Optical aberrations are deviations of optical rays that prevent the rays from being focused into a single point. From the viewpoint of wave optics, optical aberrations are deviations of a wavefront of an optical beam from exactly planar or spherical wavefront. These deviations prevent the optical beam from being focused into a tight, diffraction-limited spot. Perhaps the most common optical aberration is a spherical aberration caused by lenses having spherical surface shape(s). Spherical lenses are the easiest to produce, due to the polishing naturally resulting in spherical shape of lenses. However, spherical lenses cannot focus wide light beams into diffraction-limited spots, because a spherical surface tends to over-focus marginal rays impinging far from the lens center, and under-focus central rays, blurring the focal spot. It is this geometrical property of spherical lenses with uniform refractive index that results in the spherical aberration.
A spherical aberration can be compensated by providing a compound lens having several spherical surfaces disposed so that the spherical aberrations due to individual spherical surfaces work in opposite directions. Another method is to produce an aspherical surface, either by utilizing a computerized local polishing system, which spends pre-programmed different amount of time on the lens center and the lens peripheral areas, or by injection molding using a diamond-turned aspheric mold. Both the compound and polished-aspheric lenses are relatively expensive. Injection-molded aspheric lenses can be made less expensive than the polished-aspheric lenses when mass-produced, but at present, injection-molded aspheric lenses cannot be made to a diameter larger than approximately one centimeter, for technological reasons.
A more economical method, which also works with lens diameters as large as five to eight centimeters, includes using axial gradient-index materials. Referring to FIGS. 1A to 1D, a process of making a Gradium™ lens, manufactured by LightPath Technologies Inc, Orlando, Fla., USA, is illustrated. A few layers of glass 11 (FIG. 1A) having gradually increasing refractive indices (from bottom to top; shown with different degree of shading) are fused together to obtain an index-graded blank 12 (FIG. 1B) having a refractive index profile 19 (FIG. 1D) that varies as a function of the depth coordinate, as shown, while being uniform horizontally (that is, perpendicular to the plane of FIGS. 1A to 1D). The blank 12 is then polished using usual spherical lens polishing techniques to obtain a spherical lens 13 with an axially-graded refractive index (FIG. 1C). Due to the axially-graded refractive index, edges 14 of the spherical lens 13 do not refract light rays as strongly as they would if the refractive index were not axially graded. Due to the less strong refraction of light at the edges 14 of the spherical lens 13, the spherical aberration introduced by the spherical lens 13 is reduced.
One drawback of the lens 13 is that, although it can lessen its own spherical aberration, it usually is not made to compensate a spherical aberration of another lens. For instance, in FIG. 1B, the blank 12 is flat, and therefore it refracts optical rays in a same or similar manner as a non-graded blank of glass. When it is polished to a shape to form the lens 13, its spherical aberration compensation capability is fixed by the final polished spherical shape of the lens 13.
There are areas of technology where the amount of spherical aberration is either unknown or varies from one manufactured device to another. By way of a non-limiting example, a spherical aberration can appear in a laser gain medium, or in a laser absorber medium upon self-focusing of a laser beam due to thermal lensing or non-linear optical (Kerr) effects in that medium. Since manufacturing laser systems usually involves some degree of alignment, e.g. a translation of a lens to optimize the focusing of a laser beam in the gain or absorber medium, the resulting spherical aberration of the laser beam inevitably varies from device to device, impacting the output laser beam quality. It would be beneficial to provide an optical element or device capable to compensate this variable spherical aberration of the thermal lens. In another example, setting up or aligning an optical device can involve selecting one of several spherical lenses to choose from for use in the device. Since spherical aberration depends on focal length and refractive index of the lens, it would be advantageous to provide a spherical aberration compensator, which could be easily tuned to compensate different amount of spherical aberration.